1. Field of the Invention
The present invention relates to a passive matrix light emitting device. In particular, the present invention relates to a passive matrix light emitting device using a light emitting element represented by an organic electroluminescence (EL) element for a pixel portion.
2. Description of the Related Art
In recent years, as a flat display replacing a liquid crystal display (LCD), a light emitting device with an applied light emitting material such as organic electroluminescence (EL) attracts attention, and intensive studies are performed thereto.
FIG. 5 shows an outline of a conventional light emitting device in which digital gray scale display is performed. Here, a description will be given on an example of a light emitting device that uses an organic electroluminescent material (hereinafter, abbreviated simply as EL). The light emitting device shown in FIG. 5 has a pixel portion arranged in the center of a substrate 501 made of glass or the like. The pixel portion has light emitting elements, column signal lines, and row signal lines formed thereon. A column signal line driver circuit 502 for controlling the column signal lines is disposed on the upper side of the substrate 501. On the left side thereof is disposed a row signal line driver circuit 503 for controlling the row signal lines. Note that the column signal line driver circuit 502 and the row signal line driver circuit 503 are each composed of LSI chips, and connected to the substrate 501 through a flexible print circuit (FPC).
Referring to FIG. 5, the operation of a passive matrix light emitting device that performs digital gray scale display will be described. First, a row signal line 520 in the first row is selected. A state of being selected means here that a switch 512 is connected to GND. Next, switches 508 to 511 of the column driver circuit are turned ON. Terminals of the switches 508 to 511 on one end are connected to constant current sources 504 to 507, and terminals thereof on the other end are connected to column signal lines 516 to 519, respectively. When the switches 508 to 511 are turned ON, currents outputted from the current sources 504 to 507 flow into light emitting elements 524 to 527 via the switches 508 to 511 and column signal lines 516 to 519. Then, passing through the light emitting elements 524 to 527, the currents further pass through the switch 512 via the row signal line 520, and finally flow into GND. In this way, the light emitting elements 524 to 527 emit light in response to the flow of current therethrough. Time periods for which the switches 508 to 511 are turned ON vary from each other. Gray scale display is thus performed based on the length of time period for which the switch is turned ON. After the switches 508 to 511 are all turned OFF, the switch 512 of the row signal line driver circuit becomes in a state of VCC connection. Next, a switch 513 becomes in a state of GND connection, and this operation will be repeated. In a case where a switch of the row signal line driver circuit is in VCC connection, a light emitting element of the row interested is applied with a reverse bias, so that no current flows, and no light is emitted.
The brightness of light emitting elements 524 to 539, that is, the amount of current flowing in the light emitting elements 524 to 539 can be respectively controlled by the current value of the constant current sources 504 to 507 of the column signal line driver circuit and the length of time period for which the switches 508 to 511 are turned ON. FIG. 6 shows an example of the column signal line driver circuit. A constant voltage is first generated with a built-in constant voltage source. As the constant voltage source, a known band gap regulator or the like is used in many cases. In addition, a power source with a small temperature coefficient is used. The constant voltage generated is converted into a current by an operational amplifier 602, a transistor 603, and a resistance 604. Thus, a constant current with a small temperature coefficient can be generated. The current is reversed and duplicated to obtain plural currents by a current mirror circuit composed of transistors 605 to 609 and resistances 614 to 618, before being supplied to the column signal lines via switches 610 to 613.
Digital gray scale display of a light emitting element is described here. In the column signal line driver circuit shown in FIG. 5, if there is no variation in the length of ON time period for the switches 508 to 511, only two gray scales can be obtained in this light emitting device. A representation method of the gray scale in this light emitting device is described with reference to FIG. 7.
A timing chart of a digital time-division gray scale method is simply illustrated in FIG. 7. In this example, a frame frequency is set to 60 Hz, and 3-bit gray scale is obtained according to the time gray scale method. When the frame frequency is 60 Hz, one frame period is 16.6 ms. The value found by dividing this frame period by the number of pixels in the vertical direction approximately equals one horizontal line period. In a case where the number of pixels in the vertical direction is 220, for example, one horizontal line period takes 75 μs. In the above-mentioned method, if 90% of this horizontal line period is an image period, for which an image signal exists, the image period is 68 μs. In a case of performing 3-bit gray scale display, that is, display in eight gray scales in this image period, the length of ON time period for the switch may be set in proportion to gray scales, as illustrated in FIG. 7.
In the digital time gray scale method, the gray scale representation is conducted in the manner described above. It is of course possible to conduct the same kind of gray scale representation in a light emitting device for a color display.
Next, FIG. 14 shows an outline of a conventional light emitting device in which analog gray scale display is performed. A description will now be given on an example of a light emitting device that uses an organic EL material (hereinafter, simply abbreviated to EL). The light emitting device shown in FIG. 14 has a pixel portion arranged in the center of a substrate 1401 made of glass or the like. The pixel portion has light emitting elements, column signal lines, and row signal lines formed thereon. A column signal line driver circuit 1402 for controlling the column signal lines is disposed on the upper side of the substrate 1401. On the left side thereof is disposed a row signal line driver circuit 1403 for controlling the row signal lines. The column signal line driver circuit 1402 and the row signal line driver circuit 1403 are each composed of LSI chips, and connected to the substrate 1401 through a flexible print circuit (FPC).
Referring to FIG. 14, the operation of a passive matrix light emitting device that performs analog gray scale display will be described. First, a row signal line 1416 in the first row is selected. A state of being selected means here that a switch 1408 is connected to GND. Next, currents outputted from variable current sources 1404 to 1407 of the column driver circuit flow into light emitting elements 1420 to 1423 via column signal lines 1412 to 1415. Then, passing through the light emitting elements 1420 to 1423, the currents further pass through the switch 1408 via a row signal line 1416, and finally flow into GND. In this way, the light emitting elements 1420 to 1423 emit light in response to the flow of current therethrough. The current values of the variable current sources 1404 to 1407 are controlled in accordance with externally provided image data, and a display device performs gray scale display. After the one line period, the switch 1408 of the row signal line driver circuit becomes in a state of VCC connection. Next, a switch 1409 becomes in a state of GND connection, and this operation will be repeated. In a case where a switch of the row signal line driver circuit is in VCC connection, a light emitting element of the row interested is applied with a reverse bias, so that no current flows, and no light is emitted.
The brightness of light emitting elements 1420 to 1435, that is, the amount of current flowing in the light emitting elements 1420 to 1435 can be respectively controlled by the current value of the variable current sources 1404 to 1407 of the column signal line driver circuit. FIG. 15 shows an example of the column signal line driver circuit. First, an analog image signal is sampled by sampling switches 1509 to 1512 using sampling pulses of shift register output signals or the like. The sampled signals are retained by analog memories 1505 to 1508. When completing sampling of the signals in one line, the signals are transferred to analog memories 1521 to 1524 in response to a transfer pulse. Analog voltages thus obtained are inputted to the variable current sources composed of transistors 1501 to 1504 and resistances 1505 to 1508. The variable current sources output to the column signal lines currents corresponding to the inputted voltages.
Incidentally, problems are mentioned concerning a light emitting device using self-luminous elements such as light emitting elements. As described above, in a time period for which a light emitting element emits light, a current is always supplied and flows in the light emitting element. Therefore, if such an illumination continues for a long time, the property of light emitting element itself is degraded, which leads to the change of brightness characteristics. That is, the brightness of light emitted from a degraded light emitting element and the brightness of light from a non-degraded light emitting element vary from each other even when a current from the same current source is supplied thereto.
An explanation is made with a specific example. FIG. 10A shows a display screen of a mobile terminal device or the like using a light emitting device. In the display screen, operational icons 1001 are displayed. For the application of such a device, in many-cases, a still image is displayed most often in the screen. At this time, if the icons etc. are displayed in a color (gray scale) brighter than the background, light emitting elements in pixels of areas corresponding to the displayed icons emit light for a longer time than light emitting elements in pixels of areas for the background, and are thus degraded more quickly.
It is assumed here that the degradation of the light emitting element proceeds in the above-mentioned conditions. FIGS. 10B and 10C show display examples of the light emitting device after light emitting elements thereof are degraded. First, in such black display as shown in FIG. 10B, self-luminous elements represented by light emitting elements express the color of black in a state with no voltage applied thereto. Therefore, the degradation is not so conspicuous in the black display. On the other hand, in a case of a white display, a brightness variance occurs in the screen due to the lack of brightness in the degraded light emitting elements resulting from long time illumination (in this case, the light emitting elements in positions for displaying icons etc.) as denoted by reference numeral 1011 in FIG. 10C, even when the same current is supplied to the light emitting elements.
In order to eliminate the brightness variance, there is a method of applying more current to a degraded light emitting element. However, a current supply line is generally composed of a single wiring in a light emitting device, and also, in a driver circuit, it is difficult to form an additional circuit for changing an applied current to the light emitting element in a specific pixel among pixels arranged in matrix.
As another method to solve the problem, a method could be considered in which a light emitting element having a property coping with long time illumination is used.